This invention relates to electrolytes for a fuel cell, and more particularly to improved phosphoric acid electrolyte compositions for a phosphoric acid type fuel cell, which inhibit the growth of phosphoric acid crystals, and which have lower freezing points than electrolytes comprised only of phosphoric acid so as to prevent the electrolyte from freezing even when operation of the cell is stopped at low temperatures.
It is well known that a fuel cell produces electrical energy from a reaction of an enriched or reformed fuel and oxygen. Hydrogen is typically used as the fuel and may be obtained by reforming a methanol-water mixture in a reformer that is comprised of a catalyst and a heater for vaporizing the unreformed fuel. Air is normally the source of oxygen for the fuel cell. Oftentimes, a plurality of cell units are stacked in series so as to increase the output voltage generating capacity of the fuel cell.
One type of cell unit is comprised of an anode, a cathode, and an electrolyte matrix impregnated with an electrolyte, such as phosphoric acid, interposed between the anode and cathode. Separators are used for separating the anode of one cell unit from the cathode of an adjacent cell unit and for electrically connecting the cell units in series. The separators typically include a plurality of fuel passages on the anode side and a plurality of air passages on the cathode side of the separator. These passages act to separate the inward flow of fuel and air from each other. Once inside the fuel cell, the fuel and air react to produce electrical energy through an exothermic electrochemical reaction. In operation of the fuel cell, a reformed hydrogen-rich gaseous fuel is fed through the fuel passages into the cell where it is oxidized, releasing electrons to the anode. Air is delivered through the air passages and is reduced in the cell so as to consume electrons. As a result, the fuel cell stack generates a voltage.
As previously noted, one type of fuel cell uses phosphoric acid as the electrolyte. Aqueous phosphoric acid is generally known to be an excellent electrolyte in that it is stable but has a low vapor pressure at temperatures around 200.degree. celsius and is also a good conductor. Phosphoric acid also rejects carbon dioxide, and at temperatures around 200.degree. celsius the anode is able to withstand carbon containing impurities such as carbon monoxide in the fuel gas.
Under certain conditions, however, such as in low temperatures, strong phosphoric acid may freeze when the operation of the fuel cell is stopped. It is believed that this freezing is due to the large intermolecular force of H.sub.3 PO.sub.4 and the molecular structure of H.sub.3 PO.sub.4 wherein the PO.sub.4 radicals of the acid facilitate the stable formation of crystal lattices by forming tetrahedral structures which are interlinked by hydrogen bonds.
It is, therefore, a principal object of this invention to provide improved electrolyte compositions for a phosphoric type fuel cell that prevent the formation of phosphoric acid crystals and which do not freeze even when the operation of the cell is stopped at low temperatures.
It is a further object of this invention to provide improved electrolyte compositions for a phosphoric acid type fuel cell that do not cause any deterioration in the cell's efficiency or quality.